This invention relates to a polarizing beam splitter (PBS), and in particular to a PBS which is capable of operating over a wide wavelength region and having a large numerical aperture.
Polarizers are used in many optical systems to control the polarization state and transmission properties of light. One particular form of polarizer is known as the polarizing beam splitter (PBS) and a conventional type of PBS (known as a MacNeille type) is shown in FIG. 1. A PBS has the property that it transmits light polarized in one direction and reflects light polarized in the orthogonal direction. Usually, p-polarized light is transmitted and the s-polarized light is reflected. Such a PBS usually has a cubic shape, with the angle of incidence on the polarizing coating being 45xc2x0. The polarizing coating comprises a multi-layer stack of dielectric materials having high and low refractive indices. The dielectric coating stack is optimized to give a wide separation of the reflectance of the s- and p-polarized lights, and at the same time, maintain a large difference in their reflectance. A large difference in reflectance gives a large extinction ratio, which is an important criterion for a good PBS.
This type of PBS has the drawback, however, that the numerical aperture is small. A small numerical aperture implies a small acceptance angle and a large F-number for the optical system. Common commercial polarizing beam splitters have an acceptance angle of xc2x12xc2x0. This corresponds to an F-number of F/10. In order to achieve a larger numerical aperture, the working bandwidth has to be sacrificed. Thus it is difficult to achieve both wide bandwidth and large acceptance angle operation.
However, for many important applications it would be highly desirable to obtain a PBS with a wide bandwidth and a large numerical aperture at the same time. One of these applications is in reflective liquid crystal light valve based projection displays. Due to the divergence angle of the light source, which is typically an arc lamp, the F-number has to be small. Thus a PBS capable of operating at small F-numbers and having a large numerical aperture or large acceptance angle is needed. The following table shows the relationship between these quantities:
Li et al (L. Li, J. A. Dubrowolski, R. T. Sullivan and Z. Pang, xe2x80x9cNovel Thin Film Polarizing Beam Splitter and Its Application in High Efficiency Projection Displaysxe2x80x9d, Projection Displays V. M. H. Wu ed., SPIE Proceedings Vol. 3634, pp.52-62, 1999) described a different PBS based on birefringent materials. The design of Li et al gives an acceptance angle of more than xc2x17xc2x0 and works over the entire visible wavelength range. However, the drawback is that the angle of incidence has to be 70xc2x0 making the device rather large and difficult to use.
The present invention provides a design for a PBS that is capable of large numerical aperture and can operate over the entire visible wavelength range in order to fulfill the requirement of both broadband and large acceptance angle operations.
According to the present invention therefore there is provided a polarizing beam splitter comprising a coating sandwiched between two glass prisms, said coating comprising alternating layers of a first material having a first relatively high refractive index and a second material having a second relatively low refractive index, wherein the refractive indices of the first and second coatings, the optical thickness of each layer of the coating, and the refractive index of the glass prisms are chosen such that the coating is capable of transmitting light of a first polarization with at least 80% efficiency over a wavelength range of about 440 nm to 680 nm and reflecting light of a second polarization with at least 80% efficiency over a wavelength range of about 440 nm to 680 nm, wherein the transmission and reflectance efficiencies are maintained when the incident light is at an angle of up to xc2x15xc2x0 from an optimum incident angle.
Preferably, the transmission and reflectance efficiencies are maintained at angles of up to xc2x17xc2x0 from the optimum incident angle.
Preferably p-polarized light is transmitted and s-polarized light is reflected.
The optimum incident light angle may be selected to be 45xc2x12xc2x0 or 52xc2x12xc2x0.
In a first embodiment of the invention the coating comprises 19 layers alternately of ZnS with a refractive index of 2.35 and MgF2 with a refractive index of 1.38, and wherein said glass prisms have a refractive index of 1.69.
In a second embodiment of the invention the coating comprises 27 layers alternately of HfO2 with a refractive index of 1.98 and SiO2 with a refractive index of 1.46, and wherein said glass prisms have a refractive index of 1.52 and 64 respectively.
In a third embodiment of the invention the coating comprises 19 layers alternately of ZnS with a refractive index of 2.35 and MgF2 with a refractive index of 1.38, and wherein said glass prisms have a refractive index of 1.52.
In a fourth embodiment of the invention the coating comprises 25 layers alternately of HfO2 with a refractive index of 1.98 and SiO2 with a refractive index of 1.46, and wherein said glass prisms have a refractive index of 1.52.
According to the present invention there is further provided a LCD projector display comprising a LCD array, an input light source, and a polarizing beam splitter adapted to polarize incident light from said input light source and to direct said polarized light onto said LCD array and to transmit light reflected and repolarized from said LCD array, wherein said beam splitter comprises a coating sandwiched between two glass prisms and is adapted to transmit light of a first polarization with at least 80% efficiency over a wavelength range of about 440 nm to 680 nm and to reflect light of a second polarization with at least 80% efficiency over a wavelength range of about 440 nm to 680 nm, wherein the transmission and reflectance efficiencies are maintained when the incident light from said light source is at an angle of up to xc2x15xc2x0 from an optimum incident angle.
In a preferred arrangement the display may further comprise a pre-polarizer located between said input light source and said beam splitter, and/or a post-polarizer located between said beam splitter and an output.
Preferably the transmission and reflectance efficiencies are maintained when the incident light is at an angle of up to xc2x17xc2x0 from said optimum incident angle. Preferably p-polarized light is transmitted and s-polarized light is reflected. In preferred embodiments of the display of the present invention, the optimum incident angle is 45xc2x12xc2x0 or 52xc2x12xc2x0.
In one specific embodiment of the invention the coating comprises 19 layers alternately of ZnS with a refractive index of 2.35 and MgF2 with a refractive index of 1.38, and wherein said glass prisms have a refractive index of 1.69.
In a second specific embodiment of the invention the coating comprises 27 layers alternately of HfO2 with a refractive index of 1.98 and SiO2 with a refractive index of 1.46, and wherein said glass prisms have a refractive index of 1.52 and 1.64 respectively.
In a third specific embodiment of the invention the coating comprises 19 layers alternately of ZnS with a refractive index of 2.35 and MgF2 with a refractive index of 1.38, and wherein said glass prisms have a refractive index of 1.52.
In a fourth specific embodiment of the invention the coating comprises 25 layers alternately of HfO2 with a refractive index of 1.98 and SiO2 with a refractive index of 1.46, and wherein said glass prisms have a refractive index of 1.52.